1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus which transfers a pattern formed on a reticle (mask) onto a substrate such as a wafer has conventionally been employed to fabricate a semiconductor device using photolithography. A general projection exposure apparatus includes an illumination optical system which illuminates a reticle with a light beam from a light source, and a projection optical system which is interposed between the reticle and a wafer and projects the pattern of the reticle onto the wafer.
To obtain a uniform illumination region, an illumination optical system guides a light beam from a light source to an optical integrator such as a fly-eye lens to form a secondary source near the exit surface of the optical integrator, thereby Kohler-illuminating the reticle (or its conjugate plane) via a condenser lens.
High-quality exposure requires forming an optimal effective light source in accordance with the pattern of a reticle. The effective light source means the angular distribution of a light beam (exposure light) which enters a wafer. For example, when the light intensity distribution near the exit surface (secondary source) of an optical integrator is adjusted, it is possible to form desired effective light sources (e.g., normal illumination, annular illumination, quadrupole illumination, and dipole illumination). In addition, along with the recent increase in the NA of a projection optical system, it is becoming necessary to adjust the polarization state of exposure light. For this purpose, a polarization adjusting unit which adjusts the polarization state of exposure light is inserted in an illumination optical system.
High-quality exposure also requires high-accuracy control (dose control) to expose a photosensitive agent applied on a wafer with an appropriate dose. For this purpose, a light amount adjusting unit which adjusts the light amount is inserted in an illumination optical system of an exposure apparatus. The exposure apparatus detects the amount of a light beam reflected by a beam splitter inserted in the optical path of the illumination optical system and that of a light beam which enters a wafer, and controls the light amount adjusting unit based on the detection result. The light amount adjusting unit is formed from, for example, a plurality of neutral density filters, and can adjust the light amount by switching between them. See Japanese Patent Laid-Open No. 2001-284236 for details of this technique.
A light amount adjusting unit is placed at the subsequent of a polarization adjusting unit in an illumination optical system. To remove any light beam reflected by the neutral density filters from the optical path of the illumination optical system, the light amount adjusting unit is tilted with respect to an incident light beam. See Japanese Patent Laid-Open No. 2006-19702 for details of this technique. The amount of a light beam reflected by a beam splitter is detected by a first detector inserted in the illumination optical system or its vicinity. The amount of a light beam which enters a wafer is detected by a second detector arranged on the wafer surface (on a wafer stage which supports the wafer).
Unfortunately, a conventional exposure apparatus cannot perform high-accuracy dose control because a polarization adjusting unit, light amount adjusting unit, and beam splitter are set in an order from the light source side. The reason why the conventional exposure apparatus cannot perform high-accuracy dose control will be explained in detail below.
It is very difficult to set the reflectance (Rs) of a beam splitter with respect to an s-polarized light component equal to the reflectance (Rp) of the beam splitter with respect to a p-polarized light component. In other words, the transmittance (Ts) of the beam splitter with respect to an s-polarized light component does not become equal to the transmittance (Tp) of the beam splitter with respect to a p-polarized light component. The relationship among the Rs, Rp, Ts, and Tp values changes depending on the incident angle with respect to the beam splitter. For this reason, the angular distribution of a light beam which enters the beam splitter changes upon switching (to be referred to as “illumination mode switching” hereinafter) of the coherence factor (a) or shape of an effective light source. That is, the Rs, Rp, Ts, and Tp values of the beam splitter change for each illumination mode.
Let Is be the intensity of the s-polarized light component of a light beam which enters the beam splitter, and Ip be the intensity of its p-polarized light component. Then, an amount A of a light beam detected by the first detector and an amount B of a light beam detected by the second detector are given by:A=(Is×Rs)+(Ip×Rp)  (1)B=(Is×Ts×Cs)+(Ip×Tp×Cp)  (2)where Cs is the efficiency of an optical system from the beam splitter to the second detector with respect to an s-polarized light component, and Cp is the efficiency of the optical system from the beam splitter to the second detector with respect to a p-polarized light component. Since the environment in the exposure apparatus is controlled so that the transmittance and reflectance of the optical system are always kept constant, the Rs, Rp, Ts, Tp, Cs, and Cp values are always kept constant for each illumination mode.
High-accuracy dose control requires stabilizing the ratio between the light amount A detected by the first detector and the light amount B detected by the second detector. For this purpose, the relationship between the light amount A detected by the first detector and the light amount B detected by the second detector is calibrated for each illumination mode, and the calibration value is reflected in dose control.
Assume that the light amount adjusting unit changes the intensity Is of the s-polarized light component of a light beam which enters the beam splitter and the intensity Ip of its p-polarized light component. To maintain the ratio between the light amount A and the light amount B, it is necessary to change the intensities Is and Ip at the same rate.
Assume that the light amount adjusting unit is formed from neutral density filters. If each neutral density filter is arranged perpendicularly to an incident light beam, it has the same property irrespective of whether the incident light is s- or p-polarized. However, as described above, if each neutral density filter is tilted with respect to an incident light beam, it has different transmittances with respect to s-polarized light and p-polarized light. The difference between the transmittances with respect to s-polarized light and p-polarized light changes for each neutral density filter, so the attenuation ratio between s-polarized light and p-polarized light changes every time the neutral density filters are switched. For this reason, the ratio (i.e., the ratio between the amount of a light beam transmitted through the beam splitter and that of a light beam reflected by the beam splitter) between the intensities of the s-polarized light component and p-polarized light component of a light beam which enters the beam splitter changes. This makes it impossible to maintain the ratio between the light amount A detected by the first detector and the light amount B detected by the second detector constant.